A conventional codec comprises a decoder for converting a received digital signal to a received analog signal, and an encoder for converting a transmitting analog signal to a transmitting digital signal.
In operation, when a rising edge of a frame synchronous signal is detected, the transmitting analog signal is converted in the encoder to the transmitting digital signal having a predetermined number of bits, such that each bit is synchronous with a rising edge of each clock signal, and the received digital signal having the predetermined number of bits is converted in the decoder to the received analog signal, such that each bit of the received digital signal is synchronous with a falling edge of each clock signal.
However, the conventional codec has a disadvantage in that a significant amount of data which is processed in one frame is small, because the signal conversion is carried out only for the digital signal of the predetermined number of bits, and for the analog signal of a length corresponding to the predetermined number of bits, after the detection of the rising edge of the frame synchronous signal.
The conventional codec has a further disadvantage in that another set of an encoder and a decoder are required in conducting the signal conversion with a conversion rate which is different from the conversion rate of one bit per one clock.